1. Field of the Invention
This invention relates to a voltage regulator having an overcurrent protection circuit.
2. Description of the Related Art
It is known to provide a voltage regulator with an overcurrent protection circuit. In the event that the resistance of a load connected with the voltage regulator drops and therefore the voltage regulator is overloaded, the over-current protection circuit prevents a voltage-regulator output current from excessively increasing. The prevention of the occurrence of such an overcurrent protects the voltage regulator and the load.
Overcurrent protection circuits are of a constant-current type, a current reduction type, and a current cut-off type. The overcurrent protection circuit of the constant-current type holds a voltage-regulator output current at an acceptable constant level when a related voltage regulator is overloaded. The overcurrent protection circuit of the current reduction type decreases a voltage-regulator output current from a normal level when a related voltage regulator is overloaded. The overcurrent protection circuit of the current cut-off type interrupts a voltage-regulator output current when a related voltage regulator is overloaded.
U.S. Pat. No. 5,859,757 corresponding to Japanese patent application publication number 10-111722 discloses an output driving circuit for use in a DC stabilized power supply circuit. In U.S. Pat. No. 5,859,757, the DC stabilized power supply circuit includes an output transistor which feeds an output current to a load in response to a drive current Id. The output voltage of the DC stabilized power supply circuit, that is, the voltage across the load, is divided by a resistor network into a feedback voltage Vadj. The DC stabilized power supply circuit includes an error amplifier for generating and outputting a voltage VA representing the error of the feedback voltage Vadj from a reference voltage Vref. A base drive circuit connected between the error amplifier and the output transistor controls the drive current Id in response to the error voltage VA. The drive current Id flows through a sensing resistor. The voltage VR21 across the sensing resistor depends on the drive current Id, and indicates the output current fed to the load. A short-circuit overcurrent protecting circuit detects whether or not the output current to the load increases into an overcurrent range on the basis of the voltage VR21 across the sensing resistor. The short-circuit overcurrent protecting circuit detects whether or not the load is short-circuited on the basis of the feedback voltage Vadj. The short-circuit overcurrent protecting circuit is connected with the junction between the error amplifier and the base drive circuit. The short-circuit overcurrent protecting circuit can force the error voltage VA to drop. When the voltage VR21 across the sensing resistor exceeds a prescribed level, that is, when the output current to the load increases into the overcurrent range, the short-circuit overcurrent protecting circuit forcedly decreases the error voltage VA. The decrease in the error voltage VA causes a reduction in the drive current Id. As a result, the output current to the load decreases below the overcurrent range. When the feedback voltage Vadj drops below a given level, that is, when the load is short-circuited, the short-circuit overcurrent protecting circuit forcedly decreases the error voltage VA. As a result of the decrease in the error voltage VA, the drive current Id and also the output current to the load are limited to within acceptable ranges.
It is an object of this invention to provide a stable and power-efficient voltage regulator having an overcurrent protection circuit.
A first aspect of this invention provides a voltage regulator for converting an input voltage into a regulated voltage equal to a command level, and outputting the regulated voltage. The voltage regulator comprises a voltage detection circuit for detecting a regulator output voltage; a current detection circuit for detecting a regulator output current; a first amplifier circuit for generating a voltage error signal in response to a command output voltage level indicative of a target value of the regulator output voltage, and in response to the regulator output voltage detected by the voltage detection circuit; a second amplifier circuit for generating a current limiting signal in response to a command limit current level indicative of a limit value of the regulator output current, and in response to the regulator output current detected by the current detection circuit; means for controlling the regulator output current in response to a control current; a first transistor provided in a flow path for the control current, and being driven in response to the voltage error signal generated by the first amplifier circuit; and a second transistor provided in the flow path and connected in series with the first transistor, the second transistor being driven in response to the current limiting signal generated by the second amplifier circuit.
A second aspect of this invention is based on the first aspect thereof, and provides a voltage regulator wherein the controlling means comprises a power-supply input terminal, a power-supply output terminal, and an output transistor connected between the power-supply input terminal and the power-supply output terminal, and wherein the flow path includes a flow path for a base current through the output transistor.
A third aspect of this invention is based on the first aspect thereof, and provides a voltage regulator wherein the controlling means comprises a power-supply input terminal, a power-supply output terminal, an output transistor connected between the power-supply input terminal and the power-supply output terminal, and a drive transistor for driving the output transistor, and wherein the flow path includes a flow path for a base current through the drive transistor.
A fourth aspect of this invention is based on the second aspect thereof, and provides a voltage regulator wherein the current detection circuit comprises a resistor connected in series with the output transistor.
A fifth aspect of this invention is based on the fourth aspect thereof, and provides a voltage regulator wherein the second amplifier circuit comprises a differential amplifier circuit having first and second input terminals, the first input terminal receiving a voltage across the resistor, the second input terminal receiving a reference voltage corresponding to the command limit current level.
A sixth aspect of this invention provides a voltage regulator for converting an input voltage fed to a power-supply input terminal into a regulated voltage equal to a command level, and outputting the regulated voltage via a power-supply output terminal. The voltage regulator comprises a voltage detection circuit for detecting a regulator output voltage; a current detection circuit for detecting a regulator output current; a first amplifier circuit for generating a voltage error signal in response to a command output voltage level indicative of a target value of the regulator output voltage, and in response to the regulator output voltage detected by the voltage detection circuit; a second amplifier circuit for generating a current limiting signal in response to a command limit current level indicative of a limit value of the regulator output current, and in response to the regulator output current detected by the current detection circuit; a current feed path connected between the power-supply input terminal and the power-supply output terminal; a first transistor provided in the current feed path, and being driven in response to the voltage error signal generated by the first amplifier circuit; and a second transistor provided in the current feed path and being connected in series with the first transistor, the second transistor being driven in response to the current limiting signal generated by the second amplifier circuit.
A seventh aspect of this invention is based on the sixth aspect thereof, and provides a voltage regulator wherein the current detection circuit comprises a resistor connected in series with the first and second transistors.
An eighth aspect of this invention is based on the seventh aspect thereof, and provides a voltage regulator wherein the second amplifier circuit comprises a differential amplifier circuit having first and second input terminals, the first input terminal receiving a voltage across the resistor, the second input terminal receiving a reference voltage corresponding to the command limit current level.
A ninth aspect of this invention is based on the eighth aspect thereof, and provides a voltage regulator further comprising a voltage dividing circuit connected between the power-supply input terminal and a ground terminal for generating the reference voltage.
A tenth aspect of this invention is based on the fifth aspect thereof, and provides a voltage regulator further comprising a constant-voltage circuit connected with the power-supply input terminal for generating the reference voltage.
An eleventh aspect of this invention is based on the first aspect thereof, and provides a voltage regulator wherein the voltage detection circuit comprises a voltage dividing circuit connected between a power-supply output terminal and a ground terminal for generating a division-result voltage, and wherein the first amplifier circuit comprises a differential amplifier circuit having first and second input terminals, the first input terminal receiving the division-result voltage, the second input terminal receiving a reference voltage corresponding to the command output voltage level.
A twelfth aspect of this invention is based on the first aspect thereof, and provides a voltage regulator wherein the first and second transistors are of a same conductivity type.
A thirteenth aspect of this invention is based on the fourth aspect thereof, and provides a voltage regulator wherein the second amplifier circuit comprises a differential amplifier circuit receiving a voltage across the resistor, and means for generating an offset voltage in the differential amplifier, the offset voltage corresponding to the command limit current level.
A fourteenth aspect of this invention provides a voltage regulator comprising a power feed line for transmitting electric power toward a load; an output transistor provided in the power feed line and having a base; a control line extending from the base of the transistor for transmitting a base current from the output transistor; first and second control transistors connected in series and provided in the control line; first means for detecting a voltage of the electric power transmitted toward the load; second means for controlling the first control transistor to control the output-transistor base current in response to the voltage detected by the first means; third means for detecting a current of the electric power transmitted toward the load; and fourth means for controlling the second control transistor to control the output-transistor base current in response to the current detected by the third means.
A fifteenth aspect of this invention provides a voltage regulator comprising a power feed line for transmitting electric power toward a load; first and second control transistors connected in series and provided in the power feed line; first means for detecting a voltage of the electric power transmitted toward the load; second means for controlling the first control transistor to control the electric power transmitted toward the load in response to the voltage detected by the first means; third means for detecting a current of the electric power transmitted toward the load; and fourth means for controlling the second control transistor to control the electric power transmitted toward the load in response to the current detected by the third means.